Frequent aberrant immunoglobulin gene rearrangements in pro-B cells revealed by a bcl-xL transgene

An instructive role for Interleukin-7 receptor α in the development of human B-cell precursor leukemia

Kinase signaling fuels growth of B-cell precursor acute lymphoblastic leukemia (BCP-ALL). Yet its role in leukemia initiation is unclear and has not been shown in primary human hematopoietic cells. We previously described activating mutations in interleukin-7 receptor alpha (IL7RA) in poor-prognosis "ph-like" BCP-ALL. Here we show that expression of activated mutant IL7RA in human CD34+ hematopoietic stem and progenitor cells induces a preleukemic state in transplanted immunodeficient NOD/LtSz-scid IL2Rγnull mice, characterized by persistence of self-renewing Pro-B cells with non-productive V(D)J gene rearrangements. Preleukemic CD34+CD10highCD19+ cells evolve into BCP-ALL with spontaneously acquired Cyclin Dependent Kinase Inhibitor 2 A (CDKN2A) deletions, as commonly observed in primary human BCP-ALL. CRISPR mediated gene silencing of CDKN2A in primary human CD34+ cells transduced with activated IL7RA results in robust development of BCP-ALLs in-vivo. Thus, we demonstrate that constitutive activation of IL7RA can initiate preleukemia in primary human hematopoietic progenitors and cooperates with CDKN2A silencing in progression into BCP-ALL.

Folliculin Interacting Protein 1 Maintains Metabolic Homeostasis during B Cell Development by Modulating AMPK, mTORC1, and TFE3

Folliculin interacting protein 1 (Fnip1) is a cytoplasmic protein originally discovered through its interaction with the master metabolic sensor 5' AMP-activated protein kinase (AMPK) and Folliculin, a protein mutated in individuals with Birt-Hogg-Dubé Syndrome. In response to low energy, AMPK stimulates catabolic pathways such as autophagy to enhance energy production while inhibiting anabolic pathways regulated by the mechanistic target of rapamycin complex 1 (mTORC1). We previously found that constitutive disruption of Fnip1 in mice resulted in a lack of peripheral B cells because of a block in B cell development at the pre-B cell stage. Both AMPK and mTORC1 were activated in Fnip1-deficient B cell progenitors. In this study, we found inappropriate mTOR localization at the lysosome under nutrient-depleted conditions. Ex vivo lysine or arginine depletion resulted in increased apoptosis. Genetic inhibition of AMPK, inhibition of mTORC1, or restoration of cell viability with a Bcl-xL transgene failed to rescue B cell development in Fnip1-deficient mice. Fnip1-deficient B cell progenitors exhibited increased nuclear localization of transcription factor binding to IgHM enhancer 3 (TFE3) in developing B cells, which correlated with an increased expression of TFE3-target genes, increased lysosome numbers and function, and increased autophagic flux. These results indicate that Fnip1 modulates autophagy and energy response pathways in part through the regulation of AMPK, mTORC1, and TFE3 in B cell progenitors.

Spontaneous loss of B lineage transcription factors leads to pre-B leukemia in Ebf1+/-Bcl-xLTg mice

Early B-cell factor 1 (EBF1) plays a central role in B-cell lineage specification and commitment. Loss of this critical transcription factor is strongly associated with high-risk, relapsed and therapy-resistant B–cell-acute lymphoblastic leukemia, especially in children. However, Ebf1 haploinsufficient mice exhibit a normal lifespan. To determine whether prolonged survival of B cells would enable tumorigenesis in Ebf1 haploinsufficient animals, we generated Ebf1^+/–Bcl-x_L^Tg mice, which express the anti-apoptotic factor Bcl-x_L in B cells. Approximately half of Ebf1^+/–Bcl-x_L^Tg mice develop aggressive oligoclonal leukemia as they age, which engrafts in congenic wild-type recipients without prior conditioning. The neoplastic cells display a pre-B phenotype and express early developmental- and natural killer cell/myeloid-markers inappropriately. In addition, we found tumor cell-specific loss of several transcription factors critical for maintaining differentiation: EBF1, TCF3 and RUNX1. However, in the majority of tumors, loss of Ebf1 expression was not due to loss of heterozygosity. This is the first spontaneous mouse model of pre-B leukemia to demonstrate inappropriate expression of non-B-cell-specific genes associated with loss of Ebf1, Tcf3 and Runx1 expression.

The inflammatory role of phagocyte apoptotic pathways in rheumatic diseases

Rheumatoid arthritis affects nearly 1% of the world's population and is a debilitating autoimmune condition that can result in joint destruction. During the past decade, inflammatory functions have been described for signalling molecules classically involved in apoptotic and non-apoptotic death pathways, including, but not limited to, Toll-like receptor signalling, inflammasome activation, cytokine production, macrophage polarization and antigen citrullination. In light of these remarkable advances in the understanding of inflammatory mechanisms of the death machinery, this Review provides a snapshot of the available evidence implicating death pathways, especially within the phagocyte populations of the innate immune system, in the perpetuation of rheumatoid arthritis and other rheumatic diseases. Elevated levels of signalling mediators of both extrinsic and intrinsic apoptosis, as well as the autophagy, are observed in the joints of patients with rheumatoid arthritis. Furthermore, risk polymorphisms are present in signalling molecules of the extrinsic apoptotic and autophagy death pathways. Although research into the mechanisms underlying these pathways has made considerable progress, this Review highlights areas where further investigation is particularly needed. This exploration is critical, as new discoveries in this field could lead to the development of novel therapies for rheumatoid arthritis and other rheumatic diseases.

Antagonism of B cell enhancer networks by STAT5 drives leukemia and poor patient survival

The transcription factor STAT5 has a critical role in B cell acute lymphoblastic leukemia (B-ALL). How STAT5 mediates this effect is unclear. Here we found that activation of STAT5 worked together with defects in signaling components of the precursor to the B cell antigen receptor (pre-BCR), including defects in BLNK, BTK, PKCβ, NF-κB1 and IKAROS, to initiate B-ALL. STAT5 antagonized the transcription factors NF-κB and IKAROS by opposing regulation of shared target genes. Super-enhancers showed enrichment for STAT5 binding and were associated with an opposing network of transcription factors, including PAX5, EBF1, PU.1, IRF4 and IKAROS. Patients with a high ratio of active STAT5 to NF-κB or IKAROS had more-aggressive disease. Our studies indicate that an imbalance of two opposing transcriptional programs drives B-ALL and suggest that restoring the balance of these pathways might inhibit B-ALL.

TLR4 signals in B lymphocytes are transduced via the B cell antigen receptor and SYK

Toll-like receptors (TLRs) play an important role in immune responses to pathogens by transducing signals in innate immune cells in response to microbial products. TLRs are also expressed on B cells, and TLR signaling in B cells contributes to antibody-mediated immunity and autoimmunity. The SYK tyrosine kinase is essential for signaling from the B cell antigen receptor (BCR), and thus for antibody responses. Surprisingly, we find that it is also required for B cell survival, proliferation, and cytokine secretion in response to signaling through several TLRs. We show that treatment of B cells with lipopolysaccharide, the ligand for TLR4, results in SYK activation and that this is dependent on the BCR. Furthermore, we show that B cells lacking the BCR are also defective in TLR-induced B cell activation. Our results demonstrate that TLR4 signals through two distinct pathways, one via the BCR leading to activation of SYK, ERK, and AKT and the other through MYD88 leading to activation of NF-κB.

Generation of precursor, immature, and mature murine B1-cell lines from c-myc/bcl-xL-overexpressing pre-BI cells

Deregulated expression of c-myc and bcl-xL is long known to generate transformed B cells in humans and mice. We overexpressed these genes to induce in vitro and in vivo differentiation of fetal liver-derived mouse pre-BI cells to B1-lineage pre-BII-like, immature and mature B-cell lines, and to Ig-secreting cells. In vitro, doxycycline-controlled c-myc/bcl-xL-overexpressing CD19⁺ CD93⁺ c-kikt⁺ IgM^- pre-BI cells differentiate to and survive as CD19⁺ CD93⁺ c-kit^- IgM⁺ immature B1 cells. Timed CpG stimulation of these oncogene-overexpressing pre-B or immature B1 cells generates either CD19⁺ CD93^low c-kit^- IgM^- SLC^- pre-BII-like or IgM⁺ MHCII⁺ CD73⁺ CD80⁺ CD40⁺ mature B1-cell lines and IgM-secreting B1 cells in vitro and fixes their state of differentiation. All cell lines are clonable, but a majority of immature and mature B1-cell clones eventually reach a nonproliferating, surviving G₀ -state. Transplanted in vivo, c-myc/bcl-xL-overexpressing pre-B cells expand to mature B1 cells, and to IgM- and IgA-secreting plasmablasts and plasma cells. Within 2 months, plasmablasts have expanded most prominently in BM and spleen, indicating that the host selectively expanded development of these transformed plasma cells. The sIgM⁺ B1-cell lines and clones offer the possibility to study their roles in the development of B1-Ab repertoires, of B1-cell-mediated autoimmune diseases and of B1-cell malignancies.

Humanized Immunoglobulin Mice: Models for HIV Vaccine Testing and Studying the Broadly Neutralizing Antibody Problem

A vaccine that can effectively prevent HIV-1 transmission remains paramount to ending the HIV pandemic, but to do so, will likely need to induce broadly neutralizing antibody (bnAb) responses. A major technical hurdle toward achieving this goal has been a shortage of animal models with the ability to systematically pinpoint roadblocks to bnAb induction and to rank vaccine strategies based on their ability to stimulate bnAb development. Over the past 6 years, immunoglobulin (Ig) knock-in (KI) technology has been leveraged to express bnAbs in mice, an approach that has enabled elucidation of various B-cell tolerance mechanisms limiting bnAb production and evaluation of strategies to circumvent such processes. From these studies, in conjunction with the wealth of information recently obtained regarding the evolutionary pathways and paratopes/epitopes of multiple bnAbs, it has become clear that the very features of bnAbs desired for their function will be problematic to elicit by traditional vaccine paradigms, necessitating more iterative testing of new vaccine concepts. To meet this need, novel bnAb KI models have now been engineered to express either inferred prerearranged V(D)J exons (or unrearranged germline V, D, or J segments that can be assembled into functional rearranged V(D)J exons) encoding predecessors of mature bnAbs. One encouraging approach that has materialized from studies using such newer models is sequential administration of immunogens designed to bind progressively more mature bnAb predecessors. In this review, insights into the regulation and induction of bnAbs based on the use of KI models will be discussed, as will new Ig KI approaches for higher-throughput production and/or altering expression of bnAbs in vivo, so as to further enable vaccine-guided bnAb induction studies.

Conditional Disruption of Raptor Reveals an Essential Role for mTORC1 in B Cell Development, Survival, and Metabolism

Mechanistic target of rapamycin (mTOR) is a serine-threonine kinase that coordinates nutrient and growth factor availability with cellular growth, division, and differentiation. Studies examining the roles of mTOR signaling in immune function revealed critical roles for mTOR in regulating T cell differentiation and function. However, few studies have investigated the roles of mTOR in early B cell development. In this study, we found that mTOR is highly activated during the pro- and pre-B stages of mouse B cell development. Conditional disruption of the mTOR coactivating protein Raptor in developing mouse B cells resulted in a developmental block at the pre-B cell stage, with a corresponding lack of peripheral B cells and loss of Ag-specific Ab production. Pre-B cell survival and proliferation were significantly reduced in Raptor-deficient mice. Forced expression of a transgenic BCR or a BclxL transgene on Raptor-deficient B cells failed to rescue B cell development, suggesting that pre-BCR signaling and B cell survival are impaired in a BclxL-independent manner. Raptor-deficient pre-B cells exhibited significant decreases in oxidative phosphorylation and glycolysis, indicating that loss of mTOR signaling in B cells significantly impairs cellular metabolic capacity. Treatment of mice with rapamycin, an allosteric inhibitor of mTOR, recapitulated the early B cell developmental block. Collectively, our data reveal a previously uncharacterized role for mTOR signaling in early B cell development, survival, and metabolism.

Dynamic Bcl-xL (S49) and (S62) Phosphorylation/Dephosphorylation during Mitosis Prevents Chromosome Instability and Aneuploidy in Normal Human Diploid Fibroblasts

Bcl-xL proteins undergo dynamic phosphorylation/dephosphorylation on Ser49 and Ser62 residues during mitosis. The expression of Bcl-xL(S49A), (S62A) and dual (S49/62A) phosphorylation mutants in tumor cells lead to severe mitotic defects associated with multipolar spindle, chromosome lagging and bridging, and micro-, bi- and multi-nucleated cells. Because the above observations were made in tumor cells which already display genomic instability, we now address the question: will similar effects occur in normal human diploid cells? We studied normal human diploid BJ foreskin fibroblast cells expressing Bcl-xL (wild type), (S49A), (S49D), (S62A), (S62D) and the dual-site (S49/62A) and (S49/62D) mutants. Cells expressing S49 and/or S62 phosphorylation mutants showed reduced kinetics of cell population doubling. These effects on cell population doubling kinetics correlated with early outbreak of senescence with no impact on the cell death rate. Senescent cells displayed typical senescence-associated phenotypes including high-level of senescence-associated β-galactosidase activity, interleukin-6 (IL-6) secretion, tumor suppressor p53 and cyclin-dependent kinase inhibitor p21Waf1/Cip1 activation as well as γH2A.X-associated nuclear chromatin foci. Fluorescence in situ hybridization analysis and Giemsa-banded karyotypes revealed that the expression of Bcl-xL phosphorylation mutants in normal diploid BJ cells provoked chromosome instability and aneuploidy. These findings suggest that dynamic Bcl-xL(S49) and (S62) phosphorylation/dephosphorylation cycles are important in the maintenance of chromosome integrity during mitosis in normal cells. They could impact future strategies aiming to develop and identify compounds that could target not only the anti-apoptotic domain of Bcl-xL protein, but also its mitotic domain for cancer therapy.

Two Mutually Exclusive Local Chromatin States Drive Efficient V(D)J Recombination

Variable (V), diversity (D), and joining (J) (V(D)J) recombination is the first determinant of antigen receptor diversity. Understanding how recombination is regulated requires a comprehensive, unbiased readout of V gene usage. We have developed VDJ sequencing (VDJ-seq), a DNA-based next-generation-sequencing technique that quantitatively profiles recombination products. We reveal a 200-fold range of recombination efficiency among recombining V genes in the primary mouse Igh repertoire. We used machine learning to integrate these data with local chromatin profiles to identify combinatorial patterns of epigenetic features that associate with active V_H gene recombination. These features localize downstream of V_H genes and are excised by recombination, revealing a class of cis-regulatory element that governs recombination, distinct from expression. We detect two mutually exclusive chromatin signatures at these elements, characterized by CTCF/RAD21 and PAX5/IRF4, which segregate with the evolutionary history of associated V_H genes. Thus, local chromatin signatures downstream of V_H genes provide an essential layer of regulation that determines recombination efficiency.

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VDJ-seq enables precise quantification of antibody V(D)J recombination products

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Two distinct cis-regulatory designs characterize actively recombining V genes

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Putative recombination regulatory elements map downstream of mouse Igh V genes

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Recombination regulatory architecture reflects the V genes’ evolutionary history

Activation-Induced Cytidine Deaminase Expression in Human B Cell Precursors Is Essential for Central B Cell Tolerance

Activation-induced cytidine deaminase (AID), the enzyme-mediating class-switch recombination (CSR) and somatic hypermutation (SHM) of immunoglobulin genes, is essential for the removal of developing autoreactive B cells. How AID mediates central B cell tolerance remains unknown. We report that AID enzymes were produced in a discrete population of immature B cells that expressed recombination-activating gene 2 (RAG2), suggesting that they undergo secondary recombination to edit autoreactive antibodies. However, most AID+ immature B cells lacked anti-apoptotic MCL-1 and were deleted by apoptosis. AID inhibition using lentiviral-encoded short hairpin (sh)RNA in B cells developing in humanized mice resulted in a failure to remove autoreactive clones. Hence, B cell intrinsic AID expression mediates central B cell tolerance potentially through its RAG-coupled genotoxic activity in self-reactive immature B cells.

Lessons from gain- and loss-of-function models of pro-survival Bcl2 family proteins: implications for targeted therapy

Cell survival depends on the maintenance of mitochondrial integrity controlled by a well-balanced interplay between anti- and pro-apoptotic B cell lymphoma 2 (Bcl2) family members. Given their frequent deregulation in human pathologies, including autoimmunity and cancer, significant research efforts have increased our molecular understanding of how Bcl2 proteins control cell death. This has fostered the development of small non-peptidic compounds, so-called BH3-mimetics, that show excellent prospects of passing clinical trials and entering daily use for targeted therapy. Possible limitations in clinical application may, to a certain degree, be predicted from loss-of-function phenotypes gathered from studies using gene-modified mice that we attempt to summarize and discuss in this context.

Sox4 is required for the survival of pro-B cells

The development of mature B cells from hematopoietic stem cells is a strictly orchestrated process involving multiple regulatory genes. The transcription factor Sox4 is required for this process, but its role has not been systematically studied, and the underlying mechanisms remain unknown. To determine when and how Sox4 functions in the stepwise process of B cell development, we used mice harboring conditional null alleles for Sox4 and a Cre transgene. Sox4 deletion in hematopoietic stem cells almost entirely eliminated pro-B cells in both fetal livers and adult bone marrow, resulting in a severe deficiency in later stage B cells, including circulating mature B cells. Sox4-deficient pro-B cells, particularly those expressing the stem cell factor receptor c-Kit, readily underwent apoptosis, and even more so when c-Kit activity was inhibited by imatinib. C-Kit-expressing pro-B cells showed decreased activation of the c-Kit downstream protein Src upon Sox4 deletion. Likewise, the level of the anti-apoptotic Bcl2 protein was decreased in residual pro-B cells, and its restoration using a Bcl2 transgene allowed not only partial rescue of pro-B cell survival but also B cell maturation in the absence of Sox4. Our findings indicate that Sox4 is required for the survival of pro-B cells and may functionally interact with c-Kit and Bcl2.

SOX4 enables oncogenic survival signals in acute lymphoblastic leukemia

The Sox4 transcription factor mediates early B-cell differentiation. Compared with normal pre-B cells, SOX4 promoter regions in Ph(+) ALL cells are significantly hypomethylated. Loss and gain-of-function experiments identified Sox4 as a critical activator of PI3K/AKT and MAPK signaling in ALL cells. ChIP experiments confirmed that SOX4 binds to and transcriptionally activates promoters of multiple components within the PI3K/AKT and MAPK signaling pathways. Cre-mediated deletion of Sox4 had little effect on normal pre-B cells but compromised proliferation and viability of leukemia cells, which was rescued by BCL2L1 and constitutively active AKT and p110 PI3K. Consistent with these findings, high levels of SOX4 expression in ALL cells at the time of diagnosis predicted poor outcome in a pediatric clinical trial (COG P9906). Collectively, these studies identify SOX4 as a central mediator of oncogenic PI3K/AKT and MAPK signaling in ALL.

Transcription factor Ebf1 regulates differentiation stage-specific signaling, proliferation, and survival of B cells

The transcription factor Ebf1 is an important determinant of early B lymphopoiesis. To gain insight into the functions of Ebf1 at distinct stages of differentiation, we conditionally inactivated Ebf1. We found that Ebf1 is required for the proliferation, survival, and signaling of pro-B cells and peripheral B-cell subsets, including B1 cells and marginal zone B cells. The proliferation defect of Ebf1-deficient pro-B cells and the impaired expression of multiple cell cycle regulators are overcome by transformation with v-Abl. The survival defect of transformed Ebf1(fl/fl) pro-B cells can be rescued by the forced expression of the Ebf1 targets c-Myb or Bcl-x(L). In mature B cells, Ebf1 deficiency interferes with signaling via the B-cell-activating factor receptor (BAFF-R)- and B-cell receptor (BCR)-dependent Akt pathways. Moreover, Ebf1 is required for germinal center formation and class switch recombination. Genome-wide analyses of Ebf1-mediated gene expression and chromatin binding indicate that Ebf1 regulates both common and distinct sets of genes in early and late stage B cells. By regulating important components of transcription factor and signaling networks, Ebf1 appears to be involved in the coordination of cell proliferation, survival, and differentiation at multiple stages of B lymphopoiesis.

Amplified B lymphocyte CD40 signaling drives regulatory B10 cell expansion in mice

BACKGROUND

Aberrant CD40 ligand (CD154) expression occurs on both T cells and B cells in human lupus patients, which is suggested to enhance B cell CD40 signaling and play a role in disease pathogenesis. Transgenic mice expressing CD154 by their B cells (CD154(TG)) have an expanded spleen B cell pool and produce autoantibodies (autoAbs). CD22 deficient (CD22(-/-)) mice also produce autoAbs, and importantly, their B cells are hyper-proliferative following CD40 stimulation ex vivo. Combining these 2 genetic alterations in CD154(TG)CD22(-/-) mice was thereby predicted to intensify CD40 signaling and autoimmune disease due to autoreactive B cell expansion and/or activation.

METHODOLOGY/PRINCIPAL FINDINGS

CD154(TG)CD22(-/-) mice were assessed for their humoral immune responses and for changes in their endogenous lymphocyte subsets. Remarkably, CD154(TG)CD22(-/-) mice were not autoimmune, but instead generated minimal IgG responses against both self and foreign antigens. This paucity in IgG isotype switching occurred despite an expanded spleen B cell pool, higher serum IgM levels, and augmented ex vivo B cell proliferation. Impaired IgG responses in CD154(TG)CD22(-/-) mice were explained by a 16-fold expansion of functional, mature IL-10-competent regulatory spleen B cells (B10 cells: 26.7×10(6)±6 in CD154(TG)CD22(-/-) mice; 1.7×10(6)±0.4 in wild type mice, p<0.01), and an 11-fold expansion of B10 cells combined with their ex vivo-matured progenitors (B10+B10pro cells: 66×10(6)±3 in CD154(TG)CD22(-/-) mice; 6.1×10(6)±2 in wild type mice, p<0.01) that represented 39% of all spleen B cells.

CONCLUSIONS/SIGNIFICANCE

These results demonstrate for the first time that the IL-10-producing B10 B cell subset has the capacity to suppress IgG humoral immune responses against both foreign and self antigens. Thereby, therapeutic agents that drive regulatory B10 cell expansion in vivo may inhibit pathogenic IgG autoAb production in humans.

Selection of individual VH genes occurs at the pro-B to pre-B cell transition

B cells are subjected to selection at multiple checkpoints during their development. The selection of Ab H chains is difficult to study because of the large diversity of the CDR3. To study the selection of individual Ab H chain V region genes (V(H)), we performed CDR3 spectratyping of ∼ 75-300 rearrangements per individual V(H) in C57BL6/J mice. We measured the fraction of rearrangements that were in-frame in B cell DNA. We demonstrate that individual V(H)s have different fractions of in-frame rearrangements (IF fractions) ranging from 10 to 90% and that these IF fractions are reproducible in different mice. For most V(H)s, the IF fraction in pro-B cells approximated 33% and then shifted to the nearly final (mature) B cell value by the cycling pre-B cell stage. The frequency of high in-frame (IF) V(H) usage increased in cycling pre-B cells compared with that in pro-B cells, whereas this did not occur for low IF V(H)s. The IF fraction did not shift as much in BCR-expressing B cells and was minimally affected by L chain usage for most V(H). High IF clan II/III V(H)s share more positively charged CDR2 sequences, whereas high IF clan I J558 CDR2 sequences are diverse. These data indicate that individual V(H)s are subjected to differential selection, that V(H) IF fraction is mainly established through pre-BCR-mediated selection, that it may operate differently in clan I versus II/III V(H)s, and that it has a lasting influence on the Ab repertoire.

Noxa mediates p18INK4c cell-cycle control of homeostasis in B cells and plasma cell precursors

Inhibition of Cdk4/Cdk6 by p18(INK4c) (p18) is pivotal for generation of noncycling immunoglobulin (Ig)-secreting plasma cells (PCs). In the absence of p18, CD138(+) plasmacytoid cells continue to cycle and turnover rapidly, suggesting that p18 controls PC homeostasis. We now show that p18 selectively acts in a rare population of rapidly cycling CD138(hi)/B220(hi) intermediate PCs (iPCs). While retaining certain B-cell signatures, iPCs are poised to differentiate to end-stage PCs although the majority undergo apoptosis. p18 is dispensable for the development of the PC transcriptional circuitry, and Blimp-1 and Bcl-6 are expressed fully and mutually exclusively in individual iPCs. However, a minor proportion of iPCs express both, and they are preferentially protected by p18 or Bcl-xL overexpression, consistent with expansion of the iPC pool by Bcl-xL overexpression, or loss of proapoptotic Bim or Noxa. Expression of Noxa is induced during B-cell activation, peaks in iPCs, and selectively repressed by p18. It is required to promote apoptosis of cycling B cells, especially in the absence of p18. These findings define the first physiologic function for Noxa and suggest that by repressing Noxa, induction of G₁ arrest by p18 bypasses a homeostatic cell-cycle checkpoint in iPCs for PC differentiation.

Identification of two aberrant transcripts derived from a hybridoma with amplification of functional immunoglobulin variable genes

Murine monoclonal antibodies (mAbs) are widely used but have limitations if administered in humans. The use of chimeric or humanized mAbs can reduce immunogenicity. The first step in producing such mAbs is to clone murine variable genes from a hybridoma, but it is possible to amplify both functional and aberrant variable genes, as they coexist in the hybridoma. During the development of a murine-human chimeric antibody, we have cloned from a hybridoma the functional heavy chain variable region (V(H)) and light chain variable region (V(L)) genes of a mAb that blocks the binding of anthrax lethal factor to protective antigen. In this study, we report the detection of two aberrant transcripts from a hybridoma produced using myeloma cell line OUR-1, the development of a method to distinguish between the functional and abundant aberrant V(L) transcripts, and the origins of these aberrant genes. The aberrant V(L) gene is derived from OUR-1 cells, while the aberrant V(H) gene might derive from antibody repertoires in B cells or from gene rearrangement in the hybridoma cells. The aberrant V(H) and V(L) genes in this study may facilitate discrimination between the functional and aberrant variable genes from hybridoma cells.

Regulation of microRNA expression and abundance during lymphopoiesis

Although the cellular concentration of miRNAs is critical to their function, how miRNA expression and abundance are regulated during ontogeny is unclear. We applied miRNA-, mRNA-, and ChIP-Seq to characterize the microRNome during lymphopoiesis within the context of the transcriptome and epigenome. We show that lymphocyte-specific miRNAs are either tightly controlled by polycomb group-mediated H3K27me3 or maintained in a semi-activated epigenetic state prior to full expression. Because of miRNA biogenesis, the cellular concentration of mature miRNAs does not typically reflect transcriptional changes. However, we uncover a subset of miRNAs for which abundance is dictated by miRNA gene expression. We confirm that concentration of 5p and 3p miRNA strands depends largely on free energy properties of miRNA duplexes. Unexpectedly, we also find that miRNA strand accumulation can be developmentally regulated. Our data provide a comprehensive map of immunity's microRNome and reveal the underlying epigenetic and transcriptional forces that shape miRNA homeostasis.

B-cell homeostasis requires complementary CD22 and BLyS/BR3 survival signals

Peripheral B-cell numbers are tightly regulated by homeostatic mechanisms that influence the transitional and mature B-cell compartments and dictate the size and clonotypic diversity of the B-cell repertoire. B-lymphocyte stimulator (BLyS, a trademark of Human Genome Sciences, Inc.) plays a key role in regulating peripheral B-cell homeostasis. CD22 also promotes peripheral B-cell survival through ligand-dependent mechanisms. The B-cell subsets affected by the absence of BLyS and CD22 signals overlap, suggesting that BLyS- and CD22-mediated survival are intertwined. To examine this, the effects of BLyS insufficiency following neutralizing BLyS mAb treatment in mice also treated with CD22 ligand-blocking mAb were examined. Combined targeting of the BLyS and CD22 survival pathways led to significantly greater clearance of recirculating bone marrow, blood, marginal zone and follicular B cells than either treatment alone. Likewise, BLyS blockade further reduced bone marrow, blood and spleen B-cell numbers in CD22(-/-) mice. Notably, BLyS receptor expression and downstream signaling were normal in CD22(-/-) B cells, suggesting that CD22 does not directly alter BLyS responsiveness. CD22 survival signals were likewise intact in the absence of BLyS, as CD22 mAb treatment depleted blood B cells from mice with impaired BLyS receptor 3 (BR3) signaling. Finally, enforced BclxL expression, which rescues BR3 impairment, did not affect B-cell depletion following CD22 mAb treatment. Thus, the current studies support a model whereby CD22 and BLyS promote the survival of overlapping B-cell subsets but contribute to their maintenance through independent and complementary signaling pathways.

A Flt3- and Ras-dependent pathway primes B cell development by inducing a state of IL-7 responsiveness

Ras plays an important role in B cell development. However, the stage at which Ras governs B cell development remains unclear. Moreover, the upstream receptors and downstream effectors of Ras that govern B cell differentiation remain undefined. Using mice that express a dominant-negative form of Ras, we demonstrate that Ras-mediated signaling plays a critical role in the development of common lymphoid progenitors. This developmental block parallels that found in flt3(-/-) mice, suggesting that Flt3 is an important upstream activator of Ras in early B cell progenitors. Ras inhibition impaired proliferation of common lymphoid progenitors and pre-pro-B cells but not pro-B cells. Rather, Ras promotes STAT5-dependent pro-B cell differentiation by enhancing IL-7Ralpha levels and suppressing socs2 and socs3 expression. Our results suggest a model in which Flt3/Ras-dependent signals play a critical role in B cell development by priming early B cell progenitors for subsequent STAT5-dependent B cell differentiation.

Akt1 and Akt2 promote peripheral B-cell maturation and survival

Although the 3 isoforms of Akt regulate cell growth, proliferation, and survival in a wide variety of cell types, their role in B-cell development is unknown. We assessed B-cell maturation in the bone marrow (BM) and periphery in chimeras established with fetal liver progenitors lacking Akt1 and/or Akt2. We found that the generation of marginal zone (MZ) and B1 B cells, 2 key sources of antibacterial antibodies, was highly dependent on the combined expression of Akt1 and Akt2. In contrast, Akt1/2 deficiency did not negatively affect the generation of transitional or mature follicular B cells in the periphery or their precursors in the BM. However, Akt1/2-deficient follicular B cells exhibited a profound survival defect when forced to compete against wild-type B cells in vivo. Altogether, these studies show that Akt signaling plays a key role in peripheral B-cell maturation and survival.

The effects of c-Abl mutation on developing B cell differentiation and survival

c-Abl is a widely expressed Src family protein tyrosine kinase that is activated by chromosomal translocation in certain human leukemias. While shown in various experimental systems to regulate cell division and stress responses, its biological functions remain poorly understood. Although expressed at similar levels throughout B cell development, we found that the fraction of phosphorylated, active c-Abl peaks at the pro-B stage. We went on to perform a detailed analysis of B cell development in c-Abl-deficient mice. We confirmed a striking but variable decrease in pro- and pre-B cell numbers, a decrease in pre-B cell growth and an increase in pre-B cell apoptosis. This phenotype was not rescued by transgenic expression of a functional IgHC transgene and only partially rescued by the anti-apoptosis gene Bcl-x. Unlike their wild-type counterparts, c-Abl-deficient pre-B cells show a defect in Ca(2+) flux upon cross-linking of CD19, a co-receptor known to be involved in pre-B cell receptor signaling and failed to express CD25 on the cell surface. Despite these pre-B cell-signaling defects, selection for in-frame heavy-chain rearrangements was intact in the mutant mice. Remarkably, we were able to rescue the proliferative defect by culturing cells in vitro with large amounts of rIL-7. We conclude that c-Abl is required for normal B cell differentiation and survival.

A role for DNA polymerase mu in the emerging DJH rearrangements of the postgastrulation mouse embryo

The molecular complexes involved in the nonhomologous end-joining process that resolves recombination-activating gene (RAG)-induced double-strand breaks and results in V(D)J gene rearrangements vary during mammalian ontogeny. In the mouse, the first immunoglobulin gene rearrangements emerge during midgestation periods, but their repertoires have not been analyzed in detail. We decided to study the postgastrulation DJ(H) joints and compare them with those present in later life. The embryo DJ(H) joints differed from those observed in perinatal life by the presence of short stretches of nontemplated (N) nucleotides. Whereas most adult N nucleotides are introduced by terminal deoxynucleotidyl transferase (TdT), the embryo N nucleotides were due to the activity of the homologous DNA polymerase mu (Polmu), which was widely expressed in the early ontogeny, as shown by analysis of Polmu(-/-) embryos. Based on its DNA-dependent polymerization ability, which TdT lacks, Polmu also filled in small sequence gaps at the coding ends and contributed to the ligation of highly processed ends, frequently found in the embryo, by pairing to internal microhomology sites. These findings show that Polmu participates in the repair of early-embryo, RAG-induced double-strand breaks and subsequently may contribute to preserve the genomic stability and cellular homeostasis of lymphohematopoietic precursors during development.

Distinct functions for the transcription factor Foxo1 at various stages of B cell differentiation

The Foxo transcription factors (Foxo1, Foxo3, Foxo4) modulate cell fate decisions in diverse systems. Here we show that Foxo1-dependent gene expression was critical at multiple stages of B cell differentiation. Early deletion of Foxo1 caused a severe block at the pro-B cell stage, due to a failure to express interleukin 7 receptor α (IL-7Rα). Foxo1 inactivation in late pro-B cells resulted in an arrest at the pre-B cell stage due to a reduction in Rag1 and Rag2 expression. Deletion of Foxo1 in peripheral B cells led to fewer lymph node B cells due to reduced L-selectin expression, and failed class switch recombination due to impaired Aicda upregulation. Thus, Foxo1 regulates a transcriptional program that is essential for early B cell development and peripheral B cell function.

EBV LMP2A provides a surrogate pre-B cell receptor signal through constitutive activation of the ERK/MAPK pathway

Latent membrane protein 2A (LMP2A) of Epstein-Barr virus (EBV) provides developmental and survival signals that mimic those of a B-cell receptor (BCR). Expression of LMP2A during B-cell development results in the ability of B cells to exit the bone marrow in the absence of a BCR and persist in the periphery, where they would normally undergo apoptosis. This study extends the current knowledge of LMP2A function by examining the growth properties of bone marrow B cells from TgE LMP2A mice. Despite the lack of pre-BCR expression, bone marrow B cells from TgE LMP2A mice proliferate and survive in low concentrations of interleukin 7, similar to wild-type cells. Constitutive phosphorylation of ERK/MAPK and PI3K/Akt in TgE LMP2A bone marrow B cells is also reminiscent of signalling through the pre-BCR, altogether demonstrating that LMP2A provides a pre-BCR-like signal to developing B cells.

B cell receptor basal signaling regulates antigen-induced Ig light chain rearrangements

BCR editing in the bone marrow contributes to B cell tolerance by orchestrating secondary Ig rearrangements in self-reactive B cells. We have recently shown that loss of the BCR or a pharmacologic blockade of BCR proximal signaling pathways results in a global "back-differentiation" response in which immature B cells down-regulate genes important for the mature B cell program and up-regulate genes characteristic of earlier stages of B cell development. These observations led us to test the hypothesis that self-Ag-induced down-regulation of the BCR, and not self-Ag-induced positive signals, lead to Rag induction and hence receptor editing. Supporting this hypothesis, we found that immature B cells from xid (x-linked immunodeficiency) mice induce re-expression of a Rag2-GFP bacterial artificial chromosome reporter as well as wild-type immature B cells following Ag incubation. Incubation of immature B cells with self-Ag leads to a striking reversal in differentiation to the pro-/pre-B stage of development, consistent with the idea that back-differentiation results in the reinduction of genes required for L chain rearrangement and receptor editing. Importantly, Rag induction, the back-differentiation response to Ag, and editing in immature and pre-B cells are inhibited by a combination of phorbol ester and calcium ionophore, agents that bypass proximal signaling pathways and mimic BCR signaling. Thus, mimicking positive BCR signals actually inhibits receptor editing. These findings support a model whereby Ag-induced receptor editing is inhibited by BCR basal signaling on developing B cells; BCR down-regulation removes this basal signal, thereby initiating receptor editing.

Loss of Bcl-x in Ph+ B-ALL increases cellular proliferation and does not inhibit leukemogenesis

The kinase inhibitors imatinib mesylate and dasatinib are the preferred treatment for Philadelphia chromosome-positive (Ph+) leukemias, and they are highly successful in the chronic phase of chronic myeloid leukemia (CML). However, they are not efficient in Ph+ B-cell acute lymphoblastic leukemia (B-ALL). Ph+ leukemia cells are highly resistant to apoptosis, and evidence from cell lines and primary cells suggest Bcl-xL as a critical mediator of resistance to apoptosis: however, this concept has never been rigorously tested in an animal model. To clarify the role of Bcl-xL in Ph+ B-ALL, we generated 2 mouse models. In the first model, Ph+ B-ALL and loss of Bcl-xL expression are coinduced; in the second model, leukemia is induced with expression of Bcl-xL protein well above the levels found in wild-type lymphoblasts. Deletion of Bcl-xL did not inhibit leukemogenesis or affect apoptosis, but increased cellular proliferation. Consistent with this result, overexpression of Bcl-xL led to decreased cellular proliferation. These models reveal an unexpected role for Bcl-xL in cell-cycle entry and the proliferation of tumor cells.

Overexpression of Bcl(XL) in B cells promotes Th1 response and exacerbates collagen-induced arthritis

B cells play a pathogenic or regulatory role in many autoimmune diseases through production of autoantibodies, cytokine production, and Ag presentation. However, the mechanisms that regulate these B cell functions under different autoimmune settings remain unclear. In the current study, we found that when B cells overexpress an antiapoptotic gene, Bcl(XL), they significantly increased production of IFN-gamma and enhanced Th1 response. Consistently, Bcl-x(L) transgenic mice developed more severe and sustained collagen-induced arthritis due to the enhanced Th1 response. The production of autoantibodies in Bcl(XL) transgenic mice was comparable to that in wild-type mice. Thus, our results indicate a novel role of Bcl(XL) in regulating B cell functions and immune responses. In patients with rheumatoid arthritis, arthritogenic B cells often up-regulate Bcl(XL) expression, which may not only render B cells resistant to apoptosis but also alter the ability of the autoreactive B cells to produce cytokines and modulate the inflammatory response. This may have therapeutic implications if Bcl(XL) expression can be down-regulated in autoreactive B cells.

AID-deficient Bcl-xL transgenic mice develop delayed atypical plasma cell tumors with unusual Ig/Myc chromosomal rearrangements

Activation-induced cytidine deaminase (AID) is required for immunoglobulin (Ig) class switch recombination and somatic hypermutation, and has also been implicated in translocations between Ig switch regions and c-Myc in plasma cell tumors in mice. We asked if AID is required for accelerated tumor development in pristane-treated Bcl-xL transgenic BALB/c mice deficient in AID (pBxAicda−/−). pBxAicda^−/− mice developed tumors with a lower frequency (24 vs. 62%) and a longer mean latency (108 vs. 36 d) than AID-sufficient mice. The tumors appeared in oil granuloma tissue and did not form ascites. By interphase fluorescence in situ hybridization, six out of nine pBxAicda^−/− primary tumors had T(12;15) and one had T(6;15) chromosomal translocations. Two tumors were transplantable and established as stable cell lines. Molecular and cytogenetic analyses showed that one had an unusual unbalanced T(12;15) translocation, with IgH Cμ and Pvt-1 oriented head to tail at the breakpoint, resulting in an elevated expression of c-Myc. In contrast, the second was T(12;15) negative, but had an elevated N-Myc expression caused by a paracentric inversion of chromosome 12. Thus, novel mechanisms juxtapose Ig and Myc-family genes in AID-deficient plasma cell tumors.

The 3BP2 adapter protein is required for optimal B-cell activation and thymus-independent type 2 humoral response

3BP2 is a pleckstrin homology domain- and Src homology 2 (SH2) domain-containing adapter protein that is mutated in the rare human bone disorder cherubism and which has also been implicated in immunoreceptor signaling. However, a function for this protein has yet to be established. Here we show that mice lacking 3BP2 exhibited a perturbation in the peritoneal B1 and splenic marginal-zone B-cell compartments and diminished thymus-independent type 2 antigen response. 3BP2(-/-) B cells demonstrated a proliferation defect in response to antigen receptor cross-linking and a heightened sensitivity to B-cell receptor-induced death via a caspase-3-dependent apoptotic pathway. We show that 3BP2 binds via its SH2 domain to the CD19 signaling complex and is required for optimum Syk phosphorylation and calcium flux.

Feedback Loops, Reversals and Nonlinearities in Lymphocyte Development

Systems of differentiating cells are often regarded by experimental biologists as unidirectional processes, in which cells spend a fixed time at each successive developmental stage. However, mathematical modeling has in several cases revealed that differentiating cell systems are more complex than previously believed. For example, non-linear transitions, feedback effects, and even apparent reversals have been suggested by our studies on models for the development of lymphocytes and their receptor repertoires, and are reviewed in this paper. These studies have shown that cell population growth in developing lymphocyte subsets is usually nonlinear, as it depends on the density of cells in each compartment. Additionally, T cell development has been shown to be subject to feedback regulation by mature T cell subsets, and B cell development has been shown to include a phenotypic reflux from an advanced to an earlier developmental stage. The challenges we face in our efforts to understand how the repertoires of these cells are generated and regulated are also discussed here.

Differential Role for Cyclic AMP Response Element Binding Protein-1 in Multiple Stages of B Cell Development, Differentiation, and Survival

CREB-1 is expressed in the bone marrow and in developing B cells. To determine the role of CREB-1 in developing B cells in the bone marrow, several lines of transgenic (Tg) mice overexpressing a dominant-negative Ser(119-ala) phosphomutant CREB-1 in the bone marrow were generated. Analysis of RNA and protein revealed expression of the transgene in the bone marrow. Flow cytometric analysis of bone marrow cells from Tg mice revealed approximately 70% increase in pre-B1 (CD43(+)B220(+)CD24(+(int))) and approximately 60% decreased pre-BII (CD43(+)B220(+)CD24(++(high))) cells, indicating a developmental block in pre-BI to pre-BII transition. Consistent with this, the Tg mice showed approximately 4-fold decrease in immature and mature B cells in the bone marrow. RT-PCR analysis of RNA from Tg mice revealed increased JunB and c-Jun in pre-BII cells associated with decreased S-phase entry. Adoptive transfer of bone marrow cells into RAG-2(-/-) mice resulted in reconstitution of non-Tg but not Tg bone marrow-derived CD43(+)B220(+)CD24(high) population that is normally absent in RAG-2(-/-) mice. In the periphery, the Tg mice exhibited decreased CD21(dim)CD23(high)IgM(+) follicular B cells in the spleen and increased B1a and B1b B cells in the peritoneum. While exhibiting normal Ab responses to T-independent Ags and primary response to the T-dependent Ag DNP-keyhole limpet hemocyanin, the Tg mice exhibited severely impaired secondary Ab responses. These studies provide the first evidence for a differential role for CRE-binding proteins in multiple stages of B cell development, functional maturation, and B1 and B2 B cells.

Accessibility control of V(D)J recombination

Mammals contend with a universe of evolving pathogens by generating an enormous diversity of antigen receptors during lymphocyte development. Precursor B and T cells assemble functional immunoglobulin (Ig) and T cell receptor (TCR) genes via recombination of numerous variable (V), diversity (D), and joining (J) gene segments. Although this combinatorial process generates significant diversity, genetic reorganization is inherently dangerous. Thus, V(D)J recombination must be tightly regulated to ensure proper lymphocyte development and avoid chromosomal translocations that cause lymphoid tumors. Each genomic rearrangement is mediated by a common V(D)J recombinase that recognizes sequences flanking all antigen receptor gene segments. The specificity of V(D)J recombination is due, in large part, to changes in the accessibility of chromatin at target gene segments, which either permits or restricts access to recombinase. The chromatin configuration of antigen receptor loci is governed by the concerted action of enhancers and promoters, which function as accessibility control elements (ACEs). In general, ACEs act as conduits for transcription factors, which in turn recruit enzymes that covalently modify or remodel nucleosomes. These ACE-mediated alterations are critical for activation of gene segment transcription and for opening chromatin associated with recombinase target sequences. In this chapter, we describe advances in understanding the mechanisms that control V(D)J recombination at the level of chromatin accessibility. The discussion will focus on cis-acting regulation by ACEs, the nuclear factors that control ACE function, and the epigenetic modifications that establish recombinase accessibility.

Transgenic expression of Helios in B lineage cells alters B cell properties and promotes lymphomagenesis

Helios, a member of the Ikaros family of DNA-binding proteins, is expressed in multipotential lymphoid progenitors and throughout the T lineage. However, in most B lineage cells, Helios is not expressed, suggesting that its absence may be critical for B cell development and function. To test this possibility, transgenic mice were generated that express Helios under the control of an Ig mu enhancer. Commitment to the B cell lineage was unaltered in Helios transgenic mice, and numbers of surface IgM(+) B cells were normal in the bone marrow and spleen. However, both bone marrow and splenic B cells exhibited prolonged survival and enhanced proliferation. B cells in Helios transgenic mice were also hyperresponsive to Ag stimulation. These alterations were observed even though the concentration of ectopic Helios in B lineage cells, like that of endogenous Helios in thymocytes, was well below the concentration of Ikaros. Further evidence that ectopic Helios expression contributes to B cell abnormalities was provided by the observation that Helios transgenic mice developed metastatic lymphoma as they aged. Taken together, these results demonstrate that silencing of Helios is critical for normal B cell function.

Vav proteins regulate peripheral B-cell survival

Mice lacking all 3 Vav proteins fail to produce significant numbers of recirculating follicular or marginal zone B cells. Those B cells that do mature have shortened lifespans. The constitutive nuclear factor-kappaB (NF-κB) activity of resting naive B cells required Vav function and expression of cellular reticuloendotheliosis (c-Rel). Rel-A was reduced in Vav-deficient B cells. Furthermore, expression of the NF-κB-regulated antiapoptotic genes A1 and Bcl-2 was reduced in mature Vav-deficient B cells. Overexpression of Bcl-2 restored the number of mature follicular B cells in the spleens of Vav-deficient mice. When activated by B-cell receptor (BCR) cross-linking, Vav-deficient B cells failed to activate NF-κB. Vav proteins thus regulate an NF-κB-dependent survival signal in naive B cells and are required for NF-κB function after BCR cross-linking.

Restricted STAT5 activation dictates appropriate thymic B versus T cell lineage commitment

The molecular mechanisms regulating lymphocyte lineage commitment remain poorly characterized. To explore the role of the IL7R in this process, we generated transgenic mice that express a constitutively active form of STAT5 (STAT5b-CA), a key downstream IL7R effector, throughout lymphocyte development. STAT5b-CA mice exhibit a 40-fold increase in pro-B cells in the thymus. As documented by BrdU labeling studies, this increase is not due to enhanced B cell proliferation. Thymic pro-B cells in STAT5b-CA mice show a modest increase in cell survival ( approximately 4-fold), which correlates with bcl-x(L) expression. However, bcl-x(L) transgenic mice do not show increases in thymic B cell numbers. Thus, STAT5-dependent bcl-x(L) up-regulation and enhanced B cell survival are not sufficient to drive the thymic B cell development observed in STAT5b-CA mice. Importantly, thymic pro-B cells in STAT5b-CA mice are derived from early T cell progenitors (ETPs), suggesting that STAT5 acts by altering ETP lineage commitment. Supporting this hypothesis, STAT5 binds to the pax5 promoter in ETPs from STAT5b-CA mice and induces pax5, a master regulator of B cell development. Conversely, STAT5b-CA mice exhibit a decrease in the DN1b subset of ETPs, demonstrating that STAT5 activation inhibits early T cell differentiation or lineage commitment. On the basis of these findings, we propose that the observed expression of the IL-7R on common lymphoid progenitors, but not ETPs, results in differential STAT5 signaling within these distinct progenitor populations and thus helps ensure appropriate development of B cells and T cells in the bone marrow and thymic environments, respectively.

Activation of NF-κB promotes the transition of large, CD43+ pre-B cells to small, CD43− pre-B cells

The regulation of the transcription factor nuclear factor-kappaB (NF-kappaB) during B-cell development was examined using cells isolated from the bone marrow of transgenic mice expressing a kappaB luciferase reporter gene. The results indicate that the highest level of NF-kappaB activity is present in cells expressing the pre-B-cell receptor. Furthermore, cross-linking of Igbeta on CD43(+) pre-B cells is able to activate NF-kappaB in recombination-activating gene 1-deficient mice, preceding their further differentiation into CD43(-) pre-B cells. Expression of a dominant negative form of IkappaBalpha using a transgenic approach or by retroviral infection leads to a reduction in the number of CD43(+) pre-B cells. These data therefore indicate that activation of NF-kappaB in CD43(+) pre-B cells, as a result of signaling by the pre-B-cell receptor, facilitates the continued development of large, CD43(+) pre-B cells into small CD43(-) pre-B cells.

Basal immunoglobulin signaling actively maintains developmental stage in immature B cells

In developing B lymphocytes, a successful V(D)J heavy chain (HC) immunoglobulin (Ig) rearrangement establishes HC allelic exclusion and signals pro-B cells to advance in development to the pre-B stage. A subsequent functional light chain (LC) rearrangement then results in the surface expression of IgM at the immature B cell stage. Here we show that interruption of basal IgM signaling in immature B cells, either by the inducible deletion of surface Ig via Cre-mediated excision or by incubating cells with the tyrosine kinase inhibitor herbimycin A or the phosphatidylinositol 3-kinase inhibitor wortmannin, led to a striking “back-differentiation” of cells to an earlier stage in B cell development, characterized by the expression of pro-B cell genes. Cells undergoing this reversal in development also showed evidence of new LC gene rearrangements, suggesting an important role for basal Ig signaling in the maintenance of LC allelic exclusion. These studies identify a previously unappreciated level of plasticity in the B cell developmental program, and have important implications for our understanding of central tolerance mechanisms.

Gene rearrangement is a hallmark of B cell maturation. By interrupting basal cell signaling through the rearranged IgM receptor, immature B cells "back-differentiate" to an earlier stage in their development

Loss of Bim allows precursor B cell survival but not precursor B cell differentiation in the absence of interleukin 7

Interleukin (IL)-7 is a stromal cell-derived cytokine required for the survival, proliferation, and differentiation of B cell precursors. Members of the Bcl-2 family of proteins are known to have profound effects on lymphocyte survival, but not lymphocyte differentiation. To distinguish the relative dependence on IL-7 of B cell precursor survival versus B cell differentiation, the combined effects of lack of IL-7 and lack of the proapoptotic Bcl-2 relative, Bim, were studied. Bim is expressed to varying degrees in all B cell precursors and B cells. Lack of Bim compensated for lack of IL-7 in the survival of pro-, pre-, and immature B cells; however, lack of Bim did not substitute for the requirement for IL-7 in B cell precursor differentiation or B cell precursor proliferation. Precursor B cell survival is more dependent on sufficient levels of IL-7 than precursor B cell differentiation because the number of B cells and their precursors were reduced by half in mice heterozygous for IL-7 expression, but were restored to normal numbers in mice also lacking Bim. Hence, Bim and IL-7 work together to control the survival of B cell precursors and the number of B cells that exist in animals.

Epstein-Barr virus (EBV) LMP2A mediates B-lymphocyte survival through constitutive activation of the Ras/PI3K/Akt pathway

Epstein-Barr virus (EBV) establishes a lifelong latent infection in host B cells and is associated with the development of a variety of malignancies. The viral LMP2A protein mediates viral latency by mimicking a constitutively activated B-cell receptor (BCR). In vivo LMP2A provides developmental and survival signals to BCR-negative B cells, allowing them to survive in peripheral lymphoid organs. In this study, we have demonstrated that Ras is constitutively active in peripheral, BCR-negative B cells from LMP2A transgenic mice. Furthermore, increased expression of activated Ras correlated with elevated levels of Bcl-xL expression and a slower migrating, band-shifted form of Bcl-2. B cells from LMP2A transgenic mice were sensitive to apoptosis induction in the presence of specific inhibitors of Ras, phosphatidylinositol 3-kinase (PI3K), and Akt, indicating that LMP2A activates the Ras/PI3K/Akt pathway to mediate B-cell survival. Increased B-cell apoptosis correlated with reduced expression of Bcl-xL, suggesting that this Bcl-2 family member may be involved in apoptosis inhibition mediated by LMP2A. The ability of LMP2A to activate constitutively the Ras pathway, a common event during tumorigenesis, suggests that this viral protein plays an active role in the development of EBV-associated malignancies.

Mechanisms of selection mediated by interleukin-7, the preBCR, and hemokinin-1 during B-cell development

Many of the stromal-derived signals and factors that regulate B lymphopoiesis have been identified. We review recent evidence from our laboratory that shows that there are at least three phases during B-cell development when cells direct their own maturation, independent of stromal cells. Following the expression of the preB-cell receptor (preBCR), cells acquire the ability to proliferate in low levels of interleukin-7 (IL-7), which acts as a self-selecting mechanism to expand cells that have successfully expressed a preBCR in environments that are non-permissive to preBCR- cells. Second, the preBCR is required for a contact-mediated event between B-cell progenitors. Disruption at this stage prevents the further maturation of progenitors to the lipopolysaccharide (LPS)-responsive stage. Finally, the transition from IL-7 receptor to mature antigen receptor-based signaling is enhanced by a novel member of the tachykinin family, hemokinin-1. This series of maturation, survival, and differentiation signals is generated by B-lineage cells as they progress through developmental checkpoints on the way to becoming functionally mature cells.

Novel targeted deregulation of c-Myc cooperates with Bcl-X(L) to cause plasma cell neoplasms in mice

Deregulated expression of both Myc and Bcl-X(L) are consistent features of human plasma cell neoplasms (PCNs). To investigate whether targeted expression of Myc and Bcl-X(L) in mouse plasma cells might lead to an improved model of human PCN, we generated Myc transgenics by inserting a single-copy histidine-tagged mouse Myc gene, Myc(His), into the mouse Ig heavy-chain Calpha locus. We also generated Bcl-X(L) transgenic mice that contain a multicopy Flag-tagged mouse Bcl-x(Flag) transgene driven by the mouse Ig kappa light-chain 3' enhancer. Single-transgenic Bcl-X(L) mice remained tumor free by 380 days of age, whereas single-transgenic Myc mice developed B cell tumors infrequently (4 of 43, 9.3%). In contrast, double-transgenic Myc/Bcl-X(L) mice developed plasma cell tumors with short onset (135 days on average) and full penetrance (100% tumor incidence). These tumors produced monoclonal Ig, infiltrated the bone marrow, and contained elevated amounts of Myc(His) and Bcl-X(L)(Flag) proteins compared with the plasma cells that accumulated in large numbers in young tumor-free Myc/Bcl-X(L) mice. Our findings demonstrate that the enforced expression of Myc and Bcl-X(L) by Ig enhancers with peak activity in plasma cells generates a mouse model of human PCN that recapitulates some features of human multiple myeloma.

Fatal Acute Lymphoblastic Leukemia in Mice Transgenic for B Cell-Restricted bcl-xLand c-myc

Expression of the c-myc gene is frequently dysregulated in malignant tumors and translocations of c-myc into the Ig H chain locus are associated with Burkitt's-type lymphoma. There is indirect evidence that bcl-x, an anti-apoptotic member of the bcl-2 gene family, may also contribute to a variety of B lymphoid tumors. In this study, we show that mice transgenic for both B cell-restricted c-myc and bcl-x(L) developed aggressive, acute leukemias expressing early B lineage and stem cell surface markers. Of interest, the tumor cells proliferated and differentiated down the B cell developmental pathway following in vitro treatment with IL-7. Analysis of sorted leukemic cells from spleen indicated constitutive expression of sterile micro and kappa transcripts in combination with evidence for D-J(H) DNA rearrangements. Several B cell-specific genes were either not expressed or were expressed at low levels in primary tumor cells and were induced following culture with IL-7. IL-7 also increased V-Jkappa and V-DJ(H) rearrangements. These data demonstrate oncogenic synergy between c-myc and bcl-x(L) in a new mouse model for acute lymphoblastic leukemia. Tumors in these animals target an early stage in B cell development characterized by the expression of both B lineage and stem cell genes.

Premature termination codons do not affect the rate of splicing of neighboring introns

Introduction of a premature termination codon (PTC) into an exon of a gene can lead to nonsense-mediated decay of the mRNA, which is best characterized as a cytoplasmic event. However, increasing evidence has suggested that PTCs may also influence the nuclear processing of an RNA transcript, leading to models of nuclear surveillance perhaps involving translating nuclear ribosomes. We used quantitative RT-PCR to measure the in vivo steady-state levels of every exon-intron junction in wild-type, PTC-containing, and missense-containing precursor mRNAs of both the nonrearranging dihydrofolate reductase (DHFR) and the somatically rearranging Ig- micro genes. We find that each exon-intron junction's abundance and, therefore, the rate of intron removal, is not significantly affected by the presence of a PTC in a neighboring exon in either the DHFR or Ig- micro pre-mRNA. Similarly, the abundance of the uncleaved Ig- micro polyadenylation sites does not differ between wild-type and PTC-containing Ig- micro pre-mRNAs. Our Ig- micro data were confirmed by RNase protection analyses, and multiple cell isolates were examined to resolve differences with previously published data on steady-state pre-mRNA levels. We conclude that the presence of a PTC affects the rate of neither splicing nor the cleavage step of 3' end formation during pre-mRNA processing in the nucleus. Our results are discussed with respect to existing evidence for nuclear surveillance mechanisms.

Bcl-XL expression in stem cells facilitates engraftment and reduces the need for host conditioning during bone marrow transplantation

The prolonged survival of donor hematopoietic stem cells is crucial to the success of bone marrow transplantation. The anti-apoptotic gene Bcl-xL has been shown to promote survival of cells of the erythroid, myeloid and lymphoid lineages. To evaluate a potential therapeutic role for Bcl-xL, we used a retroviral vector to express Bcl-xL in donor cells used for murine bone marrow transplantation. We find that Bcl-xL expression in bone marrow cells facilitates hematopoietic reconstitution (as assessed by total cellularity) without altering cell differentiation. Most importantly, cells reconstituted with Bcl-xL are able to achieve high levels of donor chimerism even in non-ablative conditioning protocols in a syngeneic model of transplantation. Thus, expression of Bcl-xL by donor cells during bone marrow transplantation may provide a means to minimize host conditioning and toxicity while still achieving therapeutic degrees of mixed chimerism.

Targeted overexpression of Bcl-XL in B-lymphoid cells results in lymphoproliferative disease and plasma cell malignancies

Multiple myeloma is an incurable malignancy, and there is currently no mouse model that fully recapitulates the development and progression of the disease. We now describe a transgenic mouse that expresses a Bcl-XL transgene under the control of the 3'kappa immunoglobulin light chain enhancer, which is most active in murine B cells in late developmental stages. These mice developed nonmalignant plasma cell foci in the bone marrow and soft tissues and hyaline tubular casts in the kidneys. Median survival of the 3'KE/Bcl-XL mice was similar to littermate controls. When the 3'KE/Bcl-XL mouse was crossed to an Emu/c-Myc transgenic mouse, median survival of double transgenic progeny was 5.5 weeks. Peripheral blood and soft tissues were infiltrated with immature/mature B cells, and plasma cell lesions were identified in the bone marrow of all mice coexpressing Bcl-XL and c-Myc. These B- and plasma cell lesions demonstrated features consistent with malignancy. These results indicate that the 3'kappa immunoglobulin light chain enhancer can effectively target expression of Bcl-XL to B cells in late developmental stages, and they provide direct evidence that Bcl-XL can contribute to plasmacytomagenesis. Furthermore, this murine model serves as an important step in developing a novel genetically induced mouse model of plasma cell malignancies exhibiting bone marrow involvement.